In a conventional drilling process, wellbore pressure has to remain above a certain level to exclude formation fluids from the wellbore and/or prevent collapse of the borehole and below another level to prevent lost circulation. This pressure range is called the mud weight window (MWW). MWW is the range of values for mud density, which provides safe support to wellbore during the drilling process at a given depth. If the value of mud weight is chosen within the range of MWW, the wellbore is stable, and plastic deformation along the wellbore walls is minimized. Furthermore, with a safe mud weight selected within the MWW, mud loss is minimized.
The MWW is defined by two bounds which are generally the natural pressure limits of the wellbore in a formation. Its lower bound is the so-called shear failure gradient (SFG), which is the minimum mud weight required to keep the wellbore from plastic failure. The SFG is typically the formation pressure. The upper bound of the MWW is the so-called fracture gradient (FG), which is the maximum value of mud weight that can be achieved without inducing fracture openings in the formation. Because natural fractures usually exist within various kinds of formations, in practice, the value of minimum horizontal stress in mostly vertical wellbores is typically the value of FG.
In some environments, such as in highly geo-pressured formations (as encountered in geologically young offshore basins) or in depleted formations with reduced in-situ stresses straddled by formations still at virgin reservoir pressures, the allowable mud weight window may be very narrow, or in severe cases—nonexistent. A narrow mud weight window may require additional operations, for example, reducing penetration rates or setting of intermediate casing strings or drilling liners, which can greatly increase the total cost of the well. Consequently, if the mud weight window for a well can be widened, cost savings can prove enormous. Technologies exist to isolate pore pressure and consolidate the formation in the immediate vicinity of a wellbore. These technologies can effectively widen a mud weight window by reducing its lower bound.
In this same vein, how a well is planned and drilled depends on the size of the MWW. In the well design phase, a wide mud weight window can simplify the well trajectory, casing program, and other items in the well plan. With a wider window, total depth (TD) can be reached with fewer casing strings. Thus, a well can be spudded and the upper hole sections drilled with smaller bits while still providing the required production pipe diameter. In addition, cuttings volumes and disposal costs can be substantially reduced. Mud density, volume, and other properties can be adjusted to help reduce fluid costs and to help optimize drilling performance. Cement volume can also be reduced, and placement quality can be improved from better mud removal efficiency with optimized pump rates. The well can be drilled and casing installed and cemented more quickly. Even the required rig size may be reduced. Drilling a well with a wide mud weight window can substantially improve the capability to control the well and can result in improved borehole stability, drilling hydraulics, and borehole quality. These improvements can greatly increase ROP (rate of penetration) while reducing drilling incidents and subsequent trouble time. A wide MWW can prevent lost circulation, formation breakouts, and fluid influx. A wide window is also favorable to well control operations and to avoid having to set casing prematurely.
In practice, the MWW of a given wellbore can be estimated with either one-dimensional (1-D) analytical methods or three-dimensional (3-D) numerical finite element (FE) methods. The prior art 1-D methods determine horizontal stress components in terms of overburden stress and logging data along the wellbore trajectory, and only the information along the wellbore trajectory is used in determination of the MWW.
In the prior art 3-D finite element methods, a 3-D model of the formation is used, which model consists of a 3-D grid geometry and a 3-D mechanical constitutive relationship between points or nodes of the grid.
The advantage of the prior art 1-D analytical tools is that they are highly efficient. Their major disadvantage is that they require that several assumptions be made in selecting input data. Moreover, the input data cannot account for data that may have different values across the formation.
Because of the complex distribution of stress directions around a salt body, while many of the afore-mentioned assumptions are usually reasonable in prior art 1-D MWW analytical tools, they may not be sufficiently accurate for certain geo-structures within a formation, such as subsalt domes, rendering accurate prediction of MWW for such structure more difficult. Specifically, for wellbores passing through subsalt domes, values of MWW predicted by prior art 1-D analytical methods are significantly different from MWW values predicted by 3-D finite element methods. This is because the effective stress ratio for the formation at a salt base varies not only with the true vertical depth (TVD), but also varies with horizontal portions. Prior art 1-D analytical methods do not account for this.
In contrast to prior art 1-D methods, the advantage of the 3-D numerical method is that it can accurately calculate the geostress distribution within formations by a 3-D finite element analysis. Moreover, because of the variables that exist for effective stress ratios with respect to formations with geo-structures, such as at a salt base, these prior art 3-D methods have become the accepted standard for calculation of MWW in such cases.
One major disadvantage of the prior art 3-D methods, however, is that they are not as efficient as the prior art 1-D methods. Specifically, because prediction of MWW with 3-D finite element methods need to build submodels to key points along the proposed well trajectory, its computational cost is significantly higher than that required by a prior art 1-D MWW analytical method.
It would be desirable to provide a method for identifying a MWW for a well section, particularly those involving geologic structures such as salt domes, that has the accuracy of the prior art 3-D methods described above, but the efficiency of the prior art 1-D methods described above.